The invention relates to an installation for the heat treatment of material, for example vacuum heat treatment and subsequent hot isostatic after-treatment.
The basic principle of such an installation is described for example in DE No. 30 14 691 and in U.S. Pat. No. 4,398,702.
In a vacuum furnace, which is simultaneously designed for the application of pressure, the following method steps take place in succession, for example during the sintering of hard metal:
The parts preshaped from powder and held together by a binding agent are heated under vacuum until the binding agent escapes. This operation is called dewaxing. In the second method step, the parts are sintered at an elevated temperature. Then a further improvement in the mechanical properties of the sintered compacts is achieved by hot isostatic redensification.
Such methods and installations for carrying them out are known and belong to the prior art. They are described, for example, in the above-mentioned patents.
During the carrying out of such methods, however, problems arise which are not satisfactorily solved in the known installations. For example, since the hot isostatic redensification take place under high pressure and a high temperature, particular importance is attached to the insulation between the hot work space and the cold chamber wall. This insulation plays an important part with regard to the constancy of temperature, the energy consumption and the operational reliability. In addition, it must, on the one hand, be practically gastight in order to prevent the escape of hot gas but on the other hand it must be able to be evacuated satisfactorily for the vacuum operation.
The heat transfer from work space to the chamber-wall is effected basically by heat conduction, convection and radiation. In vacuum operation, the heat transfer is effected solely by radiation and by heat conduction by solid components. During operation with protective gas, there is also the heat conduction of the gas and, with increasing pressure, also a corresponding heat transport by convection. This means that increasing pressure causes an increasing transport of heat to the chamber-wall. If this heat transport is not kept under control and reduced, disadvantageous effects occur. These are excessive temperatures of the chamber-wall, as a result of which the life and safety of the installation are negatively influenced, excessive energy loss and adequate homogeneity of temperature in the work space of the installation.